New Canadian research suggests that exposure to commonly prescribed antibiotics increases both the incidence and severity of allergic asthma in children.

Allergic asthma affects more than 100 million people worldwide and its prevalence is increasing on average by 50 per cent every decade, particularly among children in industrialized countries. According to the Asthma Society of Canada, asthma affects at least 12 per cent of Canadian children.

Over the past decade or so a rapidly growing body of research has made it apparent that the composition of our gut microbiota - that is, the microbes that inhabit our guts - play a large role in regulating our immune systems. Anything that disrupts the delicate balance of this microscopic ecosystem has the potential to cause immune dysfunction. Not surprisingly, antibiotic drugs which wipe out bacteria in the gut, as well as at the site of any infection intended to be treated, can have significant consequences not foreseen until recently.

Researchers at the University of British Columbia have now shown that certain antibiotics that affect intestinal bacteria also had a profound impact on allergic asthma. Their results are published in the journal EMBO reports.

"It has long been suspected that kids exposed to more antibiotics – like those in developed countries – are more prone to allergic asthma," says the study's author, UBC microbiologist Brett Finlay. "Our study is the first experimental proof that shows how."

Finlay's team at UBC's Department of Microbiology and Immunology and Michael Smith Laboratories examined how two widely used broad spectrum antibiotics – streptomycin and vancomycin – affected the gut microbiota. They found that vancomycin profoundly alters the bacterial communities in the intestine and increases severity of asthma in mouse models.

The effect was only seen with young mice, however. The same antibiotics did not impact adult mice's susceptibility to asthma, indicating that early life is a critical period of establishing a healthy gut microbiota and immune system.

"Modern societal practices, such as improved sanitation methods and widespread antibiotic use, are causing the disappearance of ancestral species of bacteria in our gut that may be critical to a healthy immune system," says Finlay.

"Our study shows this is the case with certain antibiotics and allergic asthma, and the gut-lung connection is also consistent with observations that incidence of asthma has not increased significantly in developing countries where antibiotic use is less prevalent – and in turn, the gut flora is permitted to fully develop."

The study was funded in part by the Canadian Institutes of Health Research (CIHR) through the Canadian Microbiome Initiative. Marc Ouellette, a Scientific Director at CIHR, noted the importance of the team's results: "It has been recognized that microbes play an important role in human health – and we are discovering that a disruption of these bugs is associated with a number of chronic health conditions. The important results from Prof. Finlay's team confirm that giving antibiotics to young children, which disturb their normal bacterial flora, should not be taken lightly."